Marketpress Special Instructions Field

I am trying to get this special instructions message to append to the email that gets sent to the sellers (the shipping email) and to the buyer (order confirmation email).
I followed this stream: https://premium.wpmudev.org/forums/topic/special-instructions-field#post-223264
But I can't get it to appear... I don't get the last step (what file mp-default.ph???)? I added to marketpress.php and to the message form on the admin panel but it doesn't appear in either email...
-Kate